The present invention relates to transmission of sampled correlated signals, and more particularly to recording digital television signals.
One type of recording format is NRZ (non-return-to-zero), in which a high logic level or digital one is recorded using a current of a first polarity through a magnetic recording head and a low logic level or digital zero is recorded using a current of a second opposing polarity through said head. Since for the entire duration of a one or a zero, the current has the respective polarity and constant amplitude, i.e. does not return to zero current to provide a clocking bit, a high recording density is obtained. This results in lower tape usage and cost, as well as permitting smaller cassettes for the same tape length or recording time and VTR (video tape recorder). However, if a long sequence of ones or zeroes occurs, the current will maintain its particular polarity and amplitude for the duration of the sequence, resulting in a long length of constant magnetization on the tape. This makes distinguishing between successive ones or zeroes during playback difficult. A partial solution to the problem is to use a modification of NRZ recording called "NRZ-I" (non-return- to-zero-interleaved). In this type of recording, a head current transition, and thus a recorded flux transition occurs during the middle of a one signal. This transition makes ones easy to detect, and further, NRZ-I is easier to implement than pure NRZ recording. However, with NRZ-I, recording a long sequence of all zeroes, or zeroes with only a small number of ones, still makes detection of individual bits difficult.
It is therefore desirable to provide a high recording density with easy detection of successive bits.